4.x: Document supported GraalVM version for native-image

[tomas-langer]

Follow up for #8863

We currently list supported version of Java (hotspot), Maven etc., yet we are missing an explicit statement for supported GraalVM version.

We need to add this to make a clear statement (as we always support a specific version for each Helidon major version).
Keep in mind that we support Java 21 and higher even for GraalVM "hotspot". Only native-image has restricted version support.